Steel for electrical applications and novel article

ABSTRACT

A ductile non-oriented electrical sheet containing up to 0.025% carbon, 1.5 to 2.75% silicon, 1.5 to 6% aluminum with the balance iron and normal impurities, and having the combined silicon and aluminum content within the range 3 to 7.5%, and an aluminum content equal to or greater than the silicon content. Upon a controlled atmosphere anneal, a complex oxide of iron, silicon and aluminum is formed on the sheet surface which provides a high degree of electrical resistance across the surface of the sheet.

limited States Patent he itz 1451 A 1r. 110 1072 [541 STEEL FOR ELETRICAL 2,193,768 3/1940 Masumoto ..75/124 APPLICATIONS AND NOVEL T L 2,300,336 10/1942 Bozorth ..148/31.55 2,873,225 2/1959 Adams ..148/3155 [72] Inventor: L st r J- R g tz, nn T p, West- 2,970,075 1/1961 Grenoble ..l48/3l.55

moreland, Pa.

- Primary Examiner-Hyland Bizot [73] Asslgnee. United States Steel Corporation Ammey Frest C Sexton [22] Filed: Dec. 5, 1969 i 211 Appl. No.: 882,729 [57] ABSTRACT A ductile non-oriented electrical sheet containing up to Related PP Dam 0.025% carbon, 1.5 to 2.75% silicon, 1.5 to 6% aluminum with [63] continuatiommpart f Sen 59 932 4 the balance iron and normal impurities, and having the com- 1966, abandoned, bined silicon and aluminum content within the range 3 to 7.5%, and an aluminum content equal to or greater than the 52 us. Cl ..148/315, 75/124 silicon contemp a controlled atmosphere anneali a 51 lnt.Cl ..c22e39/02 p oxide of iron, siiicoii and aluminum is formed on the [58] Field of Search ..75/124, 123 5; 148/315 sheet surface which Provides a high degree of electrical sistance across the surface of the sheet. [56] References Cited 3 Claims, No Drawings UNITED STATES PATENTS 842,403 1/1907 Hadfleld ..75/124 7 STEEL FOR ELECTRICAL APPLICATIONS AND NOVEL ARTICLE This application is a continuation-in-part of application, Ser. No. 591,982, filed Nov. 4, 1966, now abandoned.

Steel for electrical sheet applications has been ferromagnetic. Ferromagnetic materials may be'divided into soft and hard ferromagnetic or permanent magnet categories. The soft ferromagnetic compositions are widely used as core materials since they are capable of being magnetized to high magnetic field densities in electrical devices such as motors, generators, power transformers, saturable core reactors, relays and similar equipment, with low energy losses incurred upon magnetization. Generally, these core materials are employed in the form of punched or sheared polycrystalline sheet laminations which are stacked to form the desired geometric shape or volume of the core. The cores may be formed from punched, sheared or slit strips of polycrystalline sheet-soft ferromagnetic material that is wound into concentric layers or stacked and bent to assume the desired geometric shape.

At the present time, a widely used material for electrical applications is a steel containing up to about silicon, the balance being substantially all iron plus the usual incidental steelmaking impurities. Alloys containing up to about 8% aluminum, balance iron, or up to 5% molybdenum, balance iron, have also been used for electrical-sheet steel applications. The alloying elements are thought to be beneficial because they increase the electrical resistivity of the material, thereby reducing eddy currents.

in electrical-sheet steel, the crystalline structure is body centered cubic at operating temperatures. Such materials may be easily magnetized in a crystallographic direction parallel to a cube edge of a unit cell of the body centered cubic lattice.

For some applications, it is desirable to employ nonoriented steels, i.e., steels in which the grains have no significant alignment or preferred orientation of their respective cube edges. Polycrystalline sheet steel satisfying this condition is characterized by having equal magnetic induction or flux density for a given magnetizing force in any direction in the polycrystalline sheet material, and therefore, an equal permeability in any direction. Unfortunately, these steels which contain high alloy content to insure low core losses are very difficult to fabricate into desirable polycrystalline sheet forms due to inability to cold roll these relatively brittle materials satisfactorily. As a result, steels containing about 3.5 to 5% silicon are generally manufactured into electrical-sheet material by a relatively expensive series of operations involving successive hot rolling and long box annealing operations. The relatively rough surface resulting from such hot rolling causes a decreased and inferior magnetic permeability. Moreover, many of these materials must be sheared since their excessive room temperature brittleness precludes economical punching for use in laminations. In addition, the brittleness of these materials limits their production to relatively short sheets that can be conveniently heated for successive hot rolling operations, or which can be manufactured into coils by joining the relatively short sheet sections end to end into coil form. The difficulties due to brittleness can be overcome by reducing the alloy content. However, while this results in a material that can be cold reduced and punched, such steels have a lower quality (higher core losses) due to the increase in eddy current losses accompanying the decrease in alloy content.

Iron-aluminum alloys also exhibit brittleness which is believed to be due to the presence of excessive, even when low, amounts of carbon. It is very difficult to remove carbon in the solid state during processing of these alloys after solidification without incurring severe and detrimental oxidation of the al- 10 it is an object of the present invention to provide a highquality, non-oriented, polycrystalline sheet steel of soft ferromagnetic material which is sufficiently ductile to permit manufacture into coil fonn yet possesses the core loss and permeability properties typical of a brittle high alloy electrihas a non-oriented body centered cubic polycrystalline structure, a preferment of orientation not greater than that available in non-oriented polycrystalline soft ferromagnetic materials and magnetic properties at least as good as such available steels, and has sufficient ductility to permit cold rolling and punching.

Another advantage of my steel composition is that the magnetic properties do not deteriorate with increased sheet thickness as much as they do with other electrical-sheet steels. Thus, the core loss properties of this steel are substantially better than conventional electrical grade steels at greater thickness as well.

An additional benefit of the steel composition provided in accordance with the invention is that it lends itself to the formation on its surface of an electrically non-conducting coating when exposed to controlled atmospheres during annealing operations. The coating so produced has a superior resistance and is desirable to further reduce core loss due to eddy currents.

In accordance with the invention, there is provided a novel steel composition suitable for ductile, nonoriented electricalsheet steel applications which consists essentially of up to about 0.025% carbon, 1.5 to 3.5% silicon, and 1.5 to 6.0% aluminum, the balance iron and normal steelmaking impurities, and the combined silicon and aluminum content being in the range of 3 to 7.5%. Another embodiment of the invention comprises a sheet of the aforementioned steel composition containing a surface coating of a complex iron-aluminum-silicon oxide, preferably less than 1 mil. Uninterrupted coatings in accordance with the invention of less than 0.0005-inch improve the surface resistance substantially.

The aluminum content of the steel compositions in accordance with the invention imparts ductility to the iron-silicon-aluminum alloy when the silicon is limited to 1.5 to 3.5%. That is to say, improved ductility will result only if the silicon content is maintained at or below the critical upper limit of 3.5%, and provided, of course, the aluminum content is at least 1.5% and the total alloy content, silicon plus aluminum, does not exceed the critical limit of 7.5%. Although these three values are indeed critical in efiecting an improved ductility, optimum ductility sufiicient to assure cold rollability can be obtained only if the silicon content is further limited to a value of 2.75% or less and to a relative amount not greater than the aluminum content. For optimum ductility, therefore, sufficient to assure cold rollability without sacrifice in high permeability and low eddy current losses it is essential that the silicon content be restricted to the range 1.5 to 2.75% with the aluminum to silicon ratio being at least equal to unity.

In summary, therefore, most of the above composition limits are very critical if the sheet is to have sufficient ductility to permit cold rollability on commercial equipment, and yet retain commercially acceptable magnetic properties. Specifically, in order to obtain the essential degree of ductility, it is critical that (1) the total alloy content, silicon and aluminum, must not exceed 7.5%; (2) the silicon content must not exceed 2.75%; and (3) the aluminum content must not be less than the silicon content. Moreover, in order to obtain magnetic properties equal to or superior to prior art commercial steels, the silicon content must be at least 1.5%. Since the aluminum content cannot be less than the silicon content, it is therefore essential that the aluminum content must also be at least 1.5%. And, as the prior art has shown, the carbon content must be minimized in order to optimize magnetic properties.

The resulting ductility effected by this invention is greater than the ductility of iron-silicon alloys of comparable alloy content. The increase in ductility is accomplished without impairing resistivity and saturation magnetization at the same time initial and maximum permeabilities increase. It is apparent that greater ductility is an important consideration in cold working these steels and the working significantly affects the final magnetic properties, particularly the core loss and permeability at given induction. Moreover, cold working is cal-sheet steel. Such steel, in accordance with the invention, more economical than hot working here. A preferred composition in accordance with the invention contains up to about 0.015% carbon, 2 to 2.75% silicon. 2 to 4% aluminum and a combined silicon and aluminum content of between 4 and 6%, the balance substantially iron and normal impurities.

in the preferred method of fabricating steel of the abovementioned composition into sheets and coils suitable for the manufacture of electrical equipment components, clean ingots of the composition are first brought to temperature of between about 2100 and 2,500 F, more desirably between 2,200 and 2350 F, by soaking and rolled to slabs, usually less than 8-inches, i.e., between 5- and 7-inches thick, at a finishing temperature between 1,600 and 2,000 F, preferably l,700 and l,800 F. The slabs may be slow cooled if desired or if necessary due to the slab thickness. The slabs are reheated to 2,l00 to 2,500 F, preferably 2,200 to 2,350 F, and hot rolled to sheet having a thickness between 0.06 and 0. 12-inch at a finishing temperature of l,600 to l,800 F. The hot-rolled sheet is water cooled by spraying to a temperature below about l,700 F, and may be cooled to room temperature.

Hot-rolled sheets so produced may then be pickled to remove scale and cold rolled to final thickness directly and without intermediate annealing, if desired. Conventional processing may also include an anneal after pickling, prior to 25 cold reduction. The cold reduced sheet may then be box annealed or continuously annealed in appropriate furnaces at between l,400 and 2,100" F, a preferred temperature between l,600 and 2,000 F is desirable, depending upon the mode of annealing. The annealing time at temperature may vary although generally annealing is performed for a time sufficient to promote recrystallization, some grain growth when desired and some purification. F -F,

An improvement in the aforementioned manufacturing process to produce a product in accordance with the invention involves perfonning the aforementioned final annealing in a hydrogen or nitrogen atmosphere, or mixtures thereof, or any inert gas atmosphere with a dew point between +30 and 26 F, and preferably less than 8 F. The dew point may be ad- Steel containing 1.5 to 6.0% aluminum, 1.5 to 3.5% silicon and the balance substantially iron and the normal steel-making impurities of the compositions shown in Table l were vacuum melted by processes which simulated melting in open hearth or basic oxygen steelmaking facilities followed by vacuum carbon deoxidizing and then cast into 3 X 8 X 14-inch ingots. After reheating to a temperature between 2,200 and 20 2,350 F, the ingots were hot rolled to l-inch slabs at a finishing temperature of between l,700 and l,750 F, and slowly cooled in insulating vermiculite. After conditioning and reheating to 2,250 F, the slabs were hot rolled to a thickness of 0.080-inch at a finishing temperature between i,600 and l,700 F. Following hot rolling, the sheet was cooled either by quenching to room temperature in a water spray or by quenching to a temperature between l,250 and l,300 F accomplished by holding in a furnace at l,200 F for i-hour. The latter process is preferred to simulate commercially employed coiling practice. The samples were then pickled, cold rolled to 0.014-inch, sheared and annealed at the various times and temperatures shown in Table I].

Sample numbers 1 through 12 were laboratory samples 35 processed as described above. Samples 13 through 16 were of TABLE I C Mn P S Si Al Cu Ni C'r Mo Sample Number:

1 AISI M14 grade. 1 AISI M15 grade 3 Maximum.

AISI M17 grade. b AISI M19 grade.

TABLE II.SAMPLE DESIGNATIQI IiII AND PROCESSING TREATMENTS EMPLOYED IN EIR PRODUCTION Annealing treatment Nominal composition, Tempercent persture, Atmos- Al Si Treatment prior to final anneal Time F, phere Sample 1 1.5 1,800 N2-15% H2 2 1,800 N2-15% H2 3 1,800 N2-15% Hz 4 1,800 Nz-15% H2 3 1,800 N2-15% H2 4 1,800 biz-16% Hz 1.5 1,800 Hz 3 1,800 Hz 1.5 1,600 N2-15% Ht 2 1,600 Nz-15% Hz 3 1,600 Nz-15% H2 4 2 3 do 1,600 Nz-15% Hz 0 4. 5 Hot roiled, commercial M14 non-oriented Hot rolled 0 4. 25 Hot roiled. commercial M15 non-oriented do 0 4 Hot rolled, commercial M17 n0n-0riented do 2. 5 2. 5 Hot roiled, slow cooled, cold reduced 4 minutes." 1,830 N2 2.5 2.5 ..d0. 3mlnutes 1,911) N;

1 Single cold reduction.

2 Simulated by slow cooling from 1,200 F, to room temperature.

3 Dou ble cold reduction with intermediat elwi Annealing was accomplished in either nitrogen, hydrogen or a mixed nitrogen atmosphere containing 3 to 15% hydrogen and having a dew point between about +6 and -26 F as shown in Table III.

TABLE III Dew Point Ranges in Experimental Anneals nealing. Alloys developed effective coatings at all annealing times and temperatures used within the dew point range of +30 to -26 F.

As mentioned above, steels in accordance with the invention exhibit substantial superior core loss properties in thicker sheet sizes than conventional electrical grade steels. The data in Table V, for example, shows how the core loss properties decrease (i.e., increase in value) with sheet thickness. Even at thicknesses of 18 and 25 mils the core loss of steel in accordance with the invention is almost 50% less than the other T t L empm m enm'h OfT'me Dew Range steels. Sample 2A corresponds in composition to sample 2 of Table I. However, it was treated by double cold reduction with :2 :1; intermediate and final anneals in an atmosphere of 15% H in 3 to 20 N2 fOl' 10 minutes at 1,800 F. 1600F 8 hr. -2 to s isoor 8 hr. -14 to 20 TABLE v 1300F 3 hr, [10 kg. core loss variation with thickness] 1400F 1 ill. -22 to -26 1400s 1 ill. -8 m -20 Thickness mm W00! 1 "2010 Composition 0. 014 0. 01s 0. 025 ieoor 1 hr. -2 to s 1800F 1 hr. -18 to 20 Sample Number: 1800F 1 hr. -4 to -20 2A 3% S1, 2% Al 0. 475 0. 534 0. 647 20o0r 1 hr. 8 m -20 E 52 4.25% S1 0.58 0.68 1600F min. l2 to -20 4.00% Si 0.65 0. 75 to 3.80% st 0.67 0. 76 0. 91 moor 10 min. -2 m -18 1830F 2.5 min. 26 1 Not available. 7 W W l900F 3 min. +20

Magnetic properties are not always well-developed in anneals conducted at a dew point of +6 F, but the surface coating was satisfactory. Dew points of less than about 8 F consistently produce superior coatings. The sheets thus produced were tested for core loss and permeability at 10 and KG and at 60 cycles and the resulting values corrected for the specific gravity of the samples being treated. The corrected data for successful combinations of composition and processing are shown in Table IV along with the results of typical torque magnetometer data and data reflecting the resistance of the surface coating as measured by the well-known Franklin insulation Test. The coating was obtained by annealing under the conditions shown in the Table for each sample. However, the surface coating could be developed through anneals at lower temperatures and shorter times with appropriate increases in the dew point of the atmosphere.

All samples containing from 1.5 to 3.5% silicon and 1.5 to 6% aluminum where the total content was from 3 to 7.5%, yielded the desired magnetic properties and ductility when processed with laboratory equipment. However, when experimentation was advanced to commercial production equipment, more limiting critical composition limits were shown to be necessary. Specifically, some compositions within the above range cannot be cold rolled on commercial equipment without breaking, despite extreme precautions to prevent breakage. Evaluation of these experiences revealed that in order to insure sufficient ductility to be cold rollable on commercial production equipment without breakage, it is necessary to confine the silicon content to the range 1.5 to 2.75%, and to assure that the aluminum content is equal to or greater than the silicon content. Therefore, even though the samples within the first discussed specified ranges do provide superior magnetic properties and improved ductility, the ductility is still not sufficiently improved to permit commercial cold rolling unless TABLE 1V.MAGNETIC PROPERTIES OF SAMPLES 10 kg. 15 kg Maximum Core Core torque, Franklin loss, loss, thousands insulation w libs Permaw libs Perrna- 01 dynesampere at nent nent emJcm. 250 p.s.i

0. 620 7, 750 1. 39 1, 200 16 0.05 0. 645 7, 500 1. 47 1, 145 20 0. 016 0.580 8,300 1. 1, 220 13 0.09 0. 640 6, 000 1. 49 450 22 0.02 0. 640 7, 500 1. 44 918 ND ND 0. 647 2, 875 1. 215 ND ND 0. 643 0, 150 1. 46 892 ND ND 0. 625 5, 850 l. 40 1, 015 ND ND 0. 51 8, 100 1. 10 1, 25 ND 0. 57 7, 570 1. 20 938 23 ND 0. 59 5, 100 1.32 857 19 ND 0. 62 5, 025 1. 31 714 20 ND 0. 532 3, 409 1. 22 378 37 0.56 0. 549 4, 379 1. 29 377 49 0.50 0. 602 4, 456 1. 34 412 57 0.47 0.519 6,897 1. 25 600 44 0.56 0.470 7, 937 1. 13 1, 230 34 0.25

Nora:

+=Atter annealing at 2,000 F. in N2-3% H2-8 F dew point, for 8 hours. ND =Not determined. +=Separate core plating and annealing operations.

+ +=Aiter annealing at 1,830" F. in nitrogen with a 26 F. dew point. +=Aiter annealing at 1,900 F. in nitrogen with a +20 F. dew point.

Annealing can be accomplished at any temperature the silicon is below 2.75%, i.e., within the range 1.5 to 2.75% between 1,400 and 2,200 F for a period of 3 minutes to 8 and the aluminumto silicon ratio is at least unity. hours as necessary to develop suitable electrical properties. It is apparent from the above that various changes may be The preferred temperature is 1,600 to 2,000 F with the opmade without departing from the invention. Accordingly, the timum conditions being between l,800 and l,900 F for 3 to scope thereof should be limited only by the appended claims 6 minutes for continuous anneal or for 8 hours for a box anwherein what is claimed is:

of from 1,400" to 2,200 F for a period of from 3 minutes to 8 hours in an atmosphere having a dew point of from +30 to -26 F, said surface coating providing a high level of resistance to the flow of electrical current through the surface of the sheet.

2. A steel according to claim 1 in which the carbon content is below about 0.015%.

3. A steel according to claim 1 in which said combined silicon and aluminum content is within the range 4.0 to 6.0%.

k a m w a UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,657 ,024 Dated April 18 1972 Inventor(s) Lester J. Regitz It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line, 33, after the period, cancel "F-F,". Column 4, Table I, under the heading "Cr", line 4, "0.05" should read 0.06 vColumn 5, Table IV, the headings "Permanent", both occurrences, should read Permeability Signed and sealed this 20th day of March 1973'.

(SEAL) Attest EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM F'O- (10-69) USCOMM DC 60376-P69 9 U S. GOVERNMENT PRINTING OFFICE: 1969 0-366-334. 

2. A steel according to claim 1 in which the carbon content is below about 0.015%.
 3. A steel according to claim 1 in which said combined silicon and aluminum content is within the range 4.0 to 6.0%. 